Proper intersection modeling
Add a Cube
Extrude its two opposite vertical side
Merge the new upper verts to the originals to create a frustum
Extrude the frustum's upper side
Select all the non horizontal edges and choose Edge>>Subdivide
Select the new edgeloop around the frustum and move it to the right position by double hit "G" and move the mouse vertically.
Extrude the appropriate faces.